Genomic analysis of a new type VI secretion system in and its implications for environmental adaptation in shrimp ponds.

The type VI secretion system (T6SS) in spp. is often used to kill heteroclonal neighbors by direct injection of toxic effectors, but its strategies in aquacultural environments receive limited attention. In this study, we conducted genomic analysis for a T6SS-harboring plasmid in strain VP157. Coculture assays were further conducted to verify its antibacterial function. The results showed that strain VP157 harbored a 132-kb plasmid, pVP157-1, which consists of two fragments: an 87.8-kb fragment identical to plasmid pTJ114-1 and a 44.2-kb T6SS gene cluster with only 4% DNA identity to T6SS1 in the reference genome. Gene-by-gene analysis of six genes representing core T6SS components suggested that each gene has distinct evolutionary origins. In vitro experimental evolution revealed that pVP157-1 can excise from the VP157 genome with an excision rate of 4%. A coculture assay suggested that strain VP157 had significantly higher antibacterial activity against and than the strain without pVP157-1(VP157). In contrast, a rapid decline was observed for the proportion of VP157 in a mock microbial community, which decreased from 10.7% to 2.1% in 5 days. The results highlighted that the acquisition of T6SS fostered the fitness of in a complex environment.